Electrochemical cells, including proton exchange membrane fuel cells, sensors, electrolyzers, and electrochemical reactors, are known in the art. Typically, the central component of such a cell is a membrane electrode assembly, comprising two catalyzing electrodes separated by an ion-conducting membrane (ICM), often referred to as a Membrane Electrode Assembly (MEA). In a fuel cell, the MEA is sandwiched between two porous, electrically-conductive backing layers to form a 5-layer assembly. When the 3-layer MEA comprises a central polymeric membrane, the fuel cell is often referred to as a polymer electrolyte fuel cell (PEFC). In a typical low-temperature fuel cell, hydrogen gas is oxidized at the anode and oxygen gas (usually as air) is reduced at the cathode: ##STR1##
Fuel cell MEAs have been constructed using catalyst electrodes in the form of applied dispersions of either Pt fines or carbon supported Pt catalysts. These conventional catalysts are applied in an ink or paste containing electrolyte to either the ICM or to a backing layer placed adjacent to the membrane. The predominant catalyst form used for hydrogen-fuel polymer electrolyte membranes is Pt or Pt alloys coated onto larger carbon particles by wet chemical methods, such as the reduction of chloroplatinic acid. This conventional form of catalyst is dispersed with ionomeric binders, solvents and often polytetrafluoroethylene (PTFE) particles, to form an ink, paste or dispersion that is applied to either the membrane, or the electrode backing material. In addition to mechanical support, it is generally believed in the art that the carbon support particles provide necessary electrical conductivity within the electrode layer.
In another variation, a catalyst metal salt is reduced in an organic solution of a solid polymer electrolyte to form a distribution of catalyst metal particles in the electrolyte, without a support particle, which is then cast onto an electrode backing layer to form the catalyst electrode.
In a further variation, Pt fines are mixed directly with a solution of solvents and polymer electrolyte and coated onto the electrode backing layer. However, because of limitations on how small the fines can be made and the stability of the dispersion, this approach results in very high, and therefore expensive, loading of the catalyst.
Conventional catalyst alloy particles are typically prepared by wet chemical or metallurgical methods and supported on conventional carbon support particles. Conventional particles have a homogeneous composition representative of the alloy stoichiometry, a generally spherical morphology indicative of the crystallite growth habit of particles produced by conventional methods, and are randomly distributed over the surface of a larger support particle. The catalyst particles may also be used without a support as a "black". Such particles are reported to be in the 2 to 25 nm size range and they increase in diameter as the amount of catalyst per support particle increases.
Various other structures and means have been used to apply or otherwise bring a catalyst in contact with an electrolyte to form electrodes. These MEAs can include: (a) porous metal films or planar distributions of metal particles or carbon supported catalyst powders deposited on the surface of the ICM; (b) metal grids or meshes deposited on or imbedded in the ICM; or (c) catalytically active nanostructured composite elements embedded in the surface of the ICM.
PEFCs are seen as a potential energy source for, e.g., electric vehicles, since PEFCs have been shown to exhibit high energy conversion efficiency, high power density and negligible pollution. In a vehicle such as an automobile, one convenient source of hydrogen gas can be the steam reformation of methanol, since methanol can be stored more easily in a vehicle than hydrogen. However, it is known that methanol reformate gas can contain as much as 25% carbon dioxide (CO.sub.2) and up to 1% carbon monoxide (CO), and that the catalytic performance of pure platinum can be significantly reduced by the presence of even 10 parts per million (ppm) of CO. Therefore, successful use of reformed hydrogen fuel depends upon either decreasing the CO content of the fuel or development of CO-tolerant catalysts, or both.
Two methods have been reported in the art to avoid the effects of CO on PEFC performance. The first method is by oxidation of CO to CO.sub.2 at the anode by means of introducing air, typically 2% by volume, into the reformate hydrogen stream, as described in U.S. Pat. No. 4,910,099. While this method is effective, it introduces added complexity to the PEFC and a loss of efficiency. The second method is to enhance the CO tolerance of Pt electrodes by alloying it with a second element, preferably ruthenium (Ru) (see, for example, M. Iwase and S. Kawatsu, Electrochemical Society Proceedings, V. 95-23, p. 12; Proceedings of the First International Symposium on Proton Conducting Membrane Fuel Cells, S. Gottesfeld, et al., Eds., The Electrochemical Society, Pennington, N.J., 1995). Tolerance for as much as 100 ppm CO was achieved for a 1:1 atomic ratio alloy of Pt:Ru on a carbon support at Pt loading level of 0.4 mg/cm.sup.2, the fuel cell operating at 80.degree. C. It is further known in the art (T. A. Zawodzinski, Jr., presented at Fuel Cells for Transportation, U.S. Department of Energy, National Laboratory R&D Meeting, Jul. 22-23, 1997, Washington, D.C.) that a PEFC having a PtRu mass loading of 0.6 mg/cm.sup.2 operating at temperatures above 100.degree. C. has been shown to be tolerant to 100 ppm CO. However, this method loses effectiveness at lower temperature operation or when lower loading of the catalyst is used.
Nanostructured composite articles have been disclosed. See, for example, U.S. Pat. Nos. 4,812,352, 5,039,561, 5,176,786, 5,336,558, 5,338,430, and 5,238,729. U.S. Pat. No. 5,338,430 discloses that nanostructured electrodes embedded in solid polymer electrolyte offer superior properties over conventional electrodes employing metal fines or carbon-supported metal catalysts, including: more efficient use of costly catalyst material and enhanced catalytic activity.